(a) Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device including interconnections having a smaller width and, more particularly, to a method for forming interconnections having a tungsten layer.
(b) Description of the Related Art
DRAM (dynamic random access memory) devices include an array of memory cells each for storing therein information. Each memory cell includes a MOSFET (metal-oxide-semiconductor field-effect-transistor) formed on a surface region of a silicon substrate and a storage capacitor overlying the MOSFET and connected thereto, and stores electric charge in the storage capacitor via the MOSFET.
Generally, DRAM devices include a plurality word lines formed on the silicon substrate with an intervention of a gate oxide film, and a plurality of bit lines overlying the word lines and extending perpendicularly thereto. The word lines control ON/OFF of the MOSFETs to receive, maintain or deliver the electric charge in the memory cell. The bit lines include a tungsten layer known as a low-resistivity metallic layer. Patent Publication JP-2002-134477A describes such a DRAM device.
In design of the conventional DRAM devices, the ratio of line width to line space of the interconnections, such as bit lines, is typically set at 1:1. Recently, a technique forming a sidewall oxide film on the side surface of the interconnections allows a via-plug connecting an overlaying interconnection to an underlying interconnection or silicon substrate to be formed between adjacent two of the interconnections, such as bit lines, in a self alignment manner. This technique necessitates reduction of the width of body of the interconnections, such as bit lines, from the conventional width by at least the thickness of the sidewall oxide film.
The line width of the bit lines, for example, is substantially at the lower limit restricted by the photolithographic technique, and it is generally difficult to drastically reduce the line width of bit lines as by improvement of the photolithographic technique.